Hypoxia inhibits Cavin-1 and Cavin-2 expression and down-regulates caveolae in adipocytes

Enhancing adipose tissue functionality in obesity: senotherapeutics, autophagy and cellular senescence as a target

Obesity, a global health crisis, disrupts multiple systemic processes, contributing to a cascade of metabolic dysfunctions by promoting the pathological expansion of visceral adipose tissue (VAT). This expansion is characterized by impaired differentiation of pre-adipocytes and an increase in senescent cells, leading to a pro-inflammatory state and exacerbated oxidative stress. Particularly, the senescence-associated secretory phenotype (SASP) and adipose tissue hypoxia further impair cellular function, promoting chronic disease development. This review delves into the potential of autophagy modulation and the therapeutic application of senolytics and senomorphics as novel strategies to mitigate adipose tissue senescence. By exploring the intricate mechanisms underlying adipocyte dysfunction and the emerging role of natural compounds in senescence modulation, we underscore the promising horizon of senotherapeutics in restoring adipose health. This approach not only offers a pathway to combat the metabolic complications of obesity, but also opens new avenues for enhancing life quality and managing the global burden of obesity-related conditions. Our analysis aims to bridge the gap between current scientific progress and clinical application, offering new perspectives on preventing and treating obesity-induced adipose dysfunction.

Novel mediators regulating angiogenesis in diabetic foot ulcer healing

A non-healing diabetic foot ulcer (DFU) is a debilitating clinical problem amounting to socioeconomic and psychosocial burdens. DFUs increase morbidity due to prolonged treatment and mortality in the case of non-treatable ulcers resulting in gangrene and septicemia. The overall amputation rate of the lower extremity with DFU ranges from 3.34% to 42.83%. Wound debridement, antibiotics, applying growth factors, negative pressure wound therapy, hyperbaric oxygen therapy, topical oxygen, and skin grafts are common therapies for DFU. However, recurrence and nonhealing ulcers are still major issues. Chronicity of inflammation, hypoxic environment, poor angiogenesis, and decreased formation of the extracellular matrix (ECM) are common impediments leading to nonhealing patterns of DFUs. Angiogenesis is crucial for wound healing since proper vessel formation facilitates nutrients, oxygen, and immune cells to the ulcer tissue to help in clearing out debris and facilitate healing. However, poor angiogenesis due to decreased expression of angiogenic mediators and matrix formation results in nonhealing and ultimately amputation. Multiple proangiogenic mediators and vascular endothelial growth factor (VEGF) therapy exist to enhance angiogenesis, but the results are not satisfactory. Thus, there is a need to investigate novel pro-angiogenic mediators that can either alone or in combination enhance the angiogenesis and healing of DFUs. In this article, we critically reviewed the existing pro-angiogenic mediators followed by potentially novel factors that might play a regulatory role in promoting angiogenesis and wound healing in DFUs.

The mechanism and biomarker function of Cavin-2 in lung ischemia-reperfusion injury

BACKGROUND

Lung Ischemia Reperfusion injury(LIRI) is one of the most predominant complications of ischemic lung disease. Cavin-2 emerged as a regulator of a variety of cellular processes, including endocytosis, lipid homeostasis, signal transduction and tumorigenesis, but the function of Cavin-2 in LIRI is unknown. The purpose of this study was to determine the predictive potential of Cavin-2 in protecting lung ischemia-reperfusion injury and its corresponding mechanisms.

METHODS

We found the strong relationship between Cavin-2 and multiple immune-related genes by deep learning method. To reveal the mechanism of Cavin-2 in LIRI, the LIRI SD rat model was constructed to detect the expression of Cavin-2 in the lung tissue of SD rats after LIRI, and the expression of Cavin-2 in lung cell lines was also detected. The expression of IL-6, IL-10 and MDA in cells after Cavin-2 over-expression or knockdown was examined under hypoxic conditions. The expression levels of p-AKT, p-STAT3 and p-ERK1/2 were measured in over-expressing Cavin-2 cells under hypoxic-ischemia conditions, and then the corresponding blockers of AKT, STAT3 and ERK1/2 were given to verify, whether they play a protective role in LIRI.

RESULTS

After hypoxia, the expression of Cavin-2 in rat lung tissues was significantly increased, and the cellular activity and IL-10 in Cavin-2 over-expressing cells were significantly higher than that of the control group, while IL-6 and MDA were significantly lower than that of the control group, while the above results were reversed in Cavin-2 knockdown cells; Meanwhile, the phosphorylation levels of AKT, STAT3, and ERK1/2 were significantly increased in Cavin-2 over-expression cells after hypoxia. When AKT, STAT3, and ERK1/2 specific blockers were given, they lost their protective effect against LIRI.

CONCLUSIONS

Cavin-2 shows biomarker potential in protecting lung from ischemia-reperfusion injury through the survivor activating factor enhancement (SAFE) and reperfusion injury salvage kinase (RISK) pathway.

UBE2O ubiquitinates PTRF/CAVIN1 and inhibits the secretion of exosome-related PTRF/CAVIN1

BACKGROUND

Exosomes are small vesicles released by cells, which have crucial functions in intercellular communication. Exosomes originated from cell membrane invagination and are released followed by multivesicular bodies (MVBs) fused with the cell membrane. It is known that Polymerase I and Transcript Release Factor (PTRF, also known as Caveolin-associated Protein-1, CAVIN1) plays an important role in caveolae formation and exosome secretion. And PTRF in exosomes has been identified as a potential biomarker in multiple malignancies such as glioma and renal cell carcinoma. However, the mechanisms of how to regulate the secretion of exosome-related PTRF remain unknown.

METHODS

We performed exogenous and endogenous immunoprecipitation assays to investigate the interaction between ubiquitin-conjugating enzyme E2O (UBE2O) and PTRF. We identified UBE2O ubiquitinated PTRF using ubiquitination assays. Then, exosomes were isolated by ultracentrifugation and identified by transmission electronic microscopy, western blot and nanoparticle tracking analysis. The effect of UBE2O on the secretion of exosome-related PTRF was analyzed by western blot, and the effect of UBE2O on exosome secretion was evaluated by exosome markers and the total protein content of exosomes.

RESULTS

Here, we showed that UBE2O interacts with PTRF directly and ubiquitinates PTRF. Functionally, we found that UBE2O inhibited the effects of PTRF on exosome secretion via decreasing caveolae formation. Importantly, UBE2O decreased exosome secretion, resulting in downregulating PTRF secretion via exosomes. Our study also identified Serum Deprivation Protein Response (SDPR, also known as Caveolin-associated Protein-2, CAVIN2) interacted with both UBE2O and PTRF. Furthermore, we found that SDPR promotes PTRF expression in exosomes. Interestingly, even in the presence of SDPR, UBE2O still inhibited the secretion of exosome-related PTRF.

CONCLUSIONS

Our study demonstrated that UBE2O downregulated exosome release and controlled the secretion of exosome-related PTRF through ubiquitinating PTRF. Since exosomes play an important role in malignant tumor growth and PTRF included in exosomes is a biomarker for several malignant tumors, increasing UBE2O expression in cells has the potential to be developed as a novel approach for cancer treatment. Video Abstract.

SGLT2 inhibitors improve kidney function and morphology by regulating renal metabolic reprogramming in mice with diabetic kidney disease

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease (ESRD) worldwide. SGLT2 inhibitors are clinically effective in halting DKD progression. However, the underlying mechanisms remain unclear. The serum and kidneys of mice with DKD were analyzed using liquid chromatography with tandem mass spectrometry (LC–MS/MS)-based metabolomic and proteomic analyses. Three groups were established: placebo-treated littermate db/m mice, placebo-treated db/db mice and EMPA-treated db/db mice. Empagliflozin (EMPA) and placebo (10 mg/kg/d) were administered for 12 weeks. EMPA treatment decreased Cys-C and urinary albumin excretion compared with placebo by 78.60% and 57.12%, respectively (p < 0.001 in all cases). Renal glomerular area, interstitial fibrosis and glomerulosclerosis were decreased by 16.47%, 68.50% and 62.82%, respectively (p < 0.05 in all cases). Multi-omic analysis revealed that EMPA treatment altered the protein and metabolic profiles in the db/db group, including 32 renal proteins, 51 serum proteins, 94 renal metabolites and 37 serum metabolites. Five EMPA-related metabolic pathways were identified by integrating proteomic and metabolomic analyses, which are involved in renal purine metabolism; pyrimidine metabolism; tryptophan metabolism; nicotinate and nicotinamide metabolism, and glycine, serine and threonine metabolism in serum. In conclusion, this study demonstrated metabolic reprogramming in mice with DKD. EMPA treatment improved kidney function and morphology by regulating metabolic reprogramming, including regulation of renal reductive stress, alleviation of mitochondrial dysfunction and reduction in renal oxidative stress reaction.

Requirement of Cavin-2 for the expression and stability of IRβ in adequate adipocyte differentiation

Objective

Adipogenesis plays an essential role in maintaining energy and hormonal balance. Cavin-2, one of the caveolae-related proteins, is abundant in adipocytes, the leading site of adipogenesis. However, the details of the roles of Cavin-2 in adipogenesis remain unknown. Here, we demonstrate the requirement of Cavin-2 for the expression and stability of IRβ in adequate adipocyte differentiation.

Methods

Cavin-2 knockout (Cavin-2 KO) and wild-type (WT) mice were fed with a high-fat diet (HFD) for 8 weeks. We evaluated body weight, food intake, and several tissues. Glucose homeostasis was assessed by glucose and insulin tolerance tests. Insulin signaling in epididymal white adipose tissue (eWAT) was determined by Akt phosphorylation. In vitro study, we evaluated adipocyte differentiation, adipogenesis-related genes, and insulin signaling to clarify the relationship between Cavin-2 and adipogenesis under the manipulation of Cavin-2 expression.

Results

Caveolae structure decreased in eWAT of Cavin-2 KO mice and Cavin-2 knockdown 3T3-L1 cells. Cavin-2 enhanced the stability of insulin receptor (IR) through direct association at the plasma membrane in adipocytes, resulting in accelerated insulin/IR/Akt signaling-induced adipogenic gene expression in insulin-containing solution-stimulated 3T3-L1 adipocytes. IR-mediated Akt activation also enhanced Cavin-2 and IR expression. Cavin-2 knockout mice showed insulin resistance with dyslipidemia and pathological hypertrophic adipocytes after a HFD.

Conclusions

Cavin-2 enhances IR stability through binding IR and regulates insulin signaling, promoting adequate adipocyte differentiation. Our findings highlight the pivotal role of Cavin-2 in adipogenesis and lipid metabolism, which may help to develop novel therapies for pathological obesity and adipogenic disorders.

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Cavin-2 expression is increased progressively during adipocyte differentiation.

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Cavin-2 knockout shows little caveolae in 3T3L-1 adipocytes and eWAT of mice.

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Cavin-2 positively regulates adipogenesis through IR stabilization.

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Cavin-2 knockout mice with a high-fat diet show insulin resistance and dyslipidemia.

Downregulation of Caveolin-1 and Upregulation of Deiodinase 3, Associated with Hypoxia-Inducible Factor-1α Increase, Are Involved in the Oxidative Stress of Graves' Orbital Adipocytes

Background: Even though the clinical features of Graves' orbitopathy (GO) are well known, its exact pathogenesis remains controversial. The imbalance of redox homeostasis in the connective tissue could play a crucial role leading to an inflammatory state and edema of soft orbital tissues, thus contributing to orbital hypoxia and increase in hypoxia-inducible factor (HIF)-1α. This oxidative stress appears to target the orbital cells such as fibroblasts and also adipocytes. This study aims to explore which pathways can lead to the aforementioned oxidative stress in GO adipose cells and therefore offers new plausible therapeutic targets. Methods: Orbital fat samples were obtained from patients with GO (Western blot [WB]: n = 8, immunohistochemistry [IHC]: n = 8) and from control patients (WB: n = 5, IHC: n = 3-5). They were processed for WB analysis and IHC of the antioxidants (catalase, superoxide dismutase 1) and for HIF-1α. The expression of caveolin-1 (Cav-1) and deiodinase 3 (DIO3), known to be regulated by HIF-1α, was also analyzed by WB and IHC, as well as the targets of Cav-1: glucose transporter type 4 (Glut-4), NADPH oxidase (NOX)-2, and endothelial nitric oxide synthase (eNOS). Triiodothyronine (T3) expression was also analyzed by IHC. Results: In GO adipocytes, the expression of catalase was reduced, whereas that of HIF-1α was strongly increased. A decreased local T3 supply was associated with DIO3 upregulation. The low expression of Cav-1 in GO adipocytes was associated not only with low expression of Glut-4 but also with an increased expression of NOX-2 and active eNOS phosphorylated on serine 1177. Conclusions: Cav-1 and DIO3, both sensitive to hypoxia and to the increase of HIF-1α, play a pivotal role in the oxidative stress in GO adipocytes. DIO3 regulates the cellular supply of T3, which is essential for the cell homeostasis. Cav-1 determines the cellular glucose supply through Glut-4 and regulates the activity of NOX-2 generating superoxide anions and that of eNOS generating nitric oxide (NO).

Energy and Dynamics of Caveolae Trafficking

Caveolae are 70–100 nm diameter plasma membrane invaginations found in abundance in adipocytes, endothelial cells, myocytes, and fibroblasts. Their bulb-shaped membrane domain is characterized and formed by specific lipid binding proteins including Caveolins, Cavins, Pacsin2, and EHD2. Likewise, an enrichment of cholesterol and other lipids makes caveolae a distinct membrane environment that supports proteins involved in cell-type specific signaling pathways. Their ability to detach from the plasma membrane and move through the cytosol has been shown to be important for lipid trafficking and metabolism. Here, we review recent concepts in caveolae trafficking and dynamics. Second, we discuss how ATP and GTP-regulated proteins including dynamin and EHD2 control caveolae behavior. Throughout, we summarize the potential physiological and cell biological roles of caveolae internalization and trafficking and highlight open questions in the field and future directions for study.

Intermittent Hypoxia Mediates Caveolae Disassembly That Parallels Insulin Resistance Development

Repetitive complete or incomplete pharyngeal collapses are leading to chronic intermittent hypoxia (CIH), a hallmark feature of obstructive sleep apnea (OSA) syndrome responsible for many metabolic disorders. In humans, an association between OSA and insulin resistance has been found independently of the degree of obesity. Based on our previous work showing that hypoxia applied to adipocytes led to cellular insulin resistance associated with caveolae flattening, we have investigated the effects of CIH on caveolae structuration in adipose tissue. Original exploratory experiences demonstrate that 6 weeks-exposure of lean mice to CIH is characterized by systemic insulin resistance and translates into adipocyte insulin signaling alterations. Chronic intermittent hypoxia also induces caveolae disassembly in white adipose tissue (WAT) illustrated by reduced plasma membrane caveolae density and enlarged caveolae width, concomitantly to WAT insulin resistance state. We show that CIH downregulates caveolar gene and protein expressions, including cavin-1, cavin-2, and EHD2, underlying molecular mechanisms responsible for such caveolae flattening. Altogether, we provide evidences for adipose tissue caveolae disassembly following CIH exposure, likely linked to cavin protein downregulation. This event may constitute the molecular basis of insulin resistance development in OSA patients.

Proteomic identification of aerobic glycolysis as a potential metabolic target for methylglyoxal in adipocytes

Obesity is often accompanied by metabolic changes in adipocytes that are closely associated with metabolic disease. Although high sugar consumption contributes to obesity, it may also directly affect adipocytes by increasing the rate of glycolysis and formation of the glycolytic by-product methylglyoxal (MG). MG is a reactive dicarbonyl that irreversibly damages proteins and other cellular components. Although the accumulation of MG is clinically associated with hyperglycemia and diabetic complications, a better understanding of how proteins are regulated by MG is needed to evaluate its role in the pathogenesis of metabolic disease. Because adipocytes rely heavily on glycolysis for glucose disposal, we hypothesized that prolonged MG treatment at nontoxic concentrations would impact the landscape of proteins involved in glucose metabolism. To test this hypothesis, we treated 3T3-L1 adipocytes with MG (100 μmol/L) and used comparative proteomics to assess the effects. We identified 25 differentially expressed proteins in adipocytes treated with MG compared to the control. Our results suggested that MG induced metabolic changes typically associated with aerobic glycolysis, including a lowered expression of proteins involved in oxidative metabolism and increased expression of the glycolytic enzymes L-lactate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase. The detection of increased lactate secreted into the culture media of adipocytes treated with MG further supported these findings, as did gene expression analysis. In summary, these results indicate MG as a metabolic contributor to aerobic glycolysis in adipocytes, a potential adaptive response to increased glucose flux which over time could lead to permanent metabolic changes.

Lessons from cavin-1 deficiency

Caveolae have been implicated in a wide range of critical physiological functions. In the past decade, the dominant role of cavin-1 in caveolae formation has been established, and it has been recognized as another master regulator for caveolae biology. Human patients with cavin-1 mutations develop lipodystrophy and muscular dystrophy and have some major pathological dysfunctions in fat tissue, skeleton muscle, heart, lung and other organs. Cavin-1 deficiency animal models consistently show similar phenotypes. However, the underlying molecular mechanisms remain to be elucidated. Recent studies have suggested many possible pathways, including mechanosensing, stress response, signal transduction, exosome secretion, and potential functions in the nucleus. Many excellent and comprehensive review articles already exist on the topics of caveolae structure formation, caveolins, and their pathophysiological functions. We will focus on recent studies using cavin-1 deficiency models, to summarize the pathophysiological changes in adipose, muscle, and other organs, followed by a summary of mechanistic studies about the roles of cavin-1, which includes caveolae formation, ribosomal RNA transcription, mechanical sensing, stress response, and exosome secretion. Further studies may help to elucidate the exact underlying molecular mechanism to explain the pathological changes observed in cavin-1 deficient human patients and animal models, so potential new therapeutic strategies can be developed.

Integrated transcriptomic correlation network analysis identifies COPD molecular determinants

Chronic obstructive pulmonary disease (COPD) is a complex and heterogeneous syndrome. Network-based analysis implemented by SWIM software can be exploited to identify key molecular switches - called "switch genes" - for the disease. Genes contributing to common biological processes or defining given cell types are usually co-regulated and co-expressed, forming expression network modules. Consistently, we found that the COPD correlation network built by SWIM consists of three well-characterized modules: one populated by switch genes, all up-regulated in COPD cases and related to the regulation of immune response, inflammatory response, and hypoxia (like TIMP1, HIF1A, SYK, LY96, BLNK and PRDX4); one populated by well-recognized immune signature genes, all up-regulated in COPD cases; one where the GWAS genes AGER and CAVIN1 are the most representative module genes, both down-regulated in COPD cases. Interestingly, 70% of AGER negative interactors are switch genes including PRDX4, whose activation strongly correlates with the activation of known COPD GWAS interactors SERPINE2, CD79A, and POUF2AF1. These results suggest that SWIM analysis can identify key network modules related to complex diseases like COPD.

Effect of hypoxia on caveolae-related protein expression and insulin signaling in adipocytes

Obesity is characterized by hypertrophy and hyperplasia of adipose tissue, which have been related to the development of hypoxia and insulin resistance. On the other hand, caveolin-1 (Cav-1), one of the main proteins of caveolae, promotes insulin receptor (IR) phosphorylation and the subsequent activation of insulin signaling. In this work we investigated the effect of hypoxia on Cav-1 regulation and the status of insulin signaling in 3T3-L1 adipocytes. Our results showed that hypoxia inhibited adipogenesis and insulin signaling in adipocytes. Furthermore, 48 h of hypoxia reduced insulin-induced glucose uptake while increased basal glucose uptake. This result was consistent with the upregulation of glucose transporter GLUT1 and the downregulation of GLUT4, which also showed defective translocation to plasma membrane when adipocytes were stimulated with insulin. In addition, the expression of caveolae-related proteins was reduced by hypoxia and chromatin immunoprecipitation assay demonstrated that Cav-1 transcription was directly regulated by HIF-1. These results strengthen the role of caveolae in insulin signaling and help to explain adipocyte response to hypoxia.

Injury induced expression of caveolar proteins in human kidney tubules - role of megakaryoblastic leukemia 1

Background

Caveolae are membrane invaginations measuring 50–100 nm. These organelles, composed of caveolin and cavin proteins, are important for cellular signaling and survival. Caveolae play incompletely defined roles in human kidneys. Induction of caveolin-1/CAV1 in diseased tubules has been described previously, but the responsible mechanism remains to be defined.

Methods

Healthy and atrophying human kidneys were stained for caveolar proteins, (caveolin 1–3 and cavin 1–4) and examined by electron microscopy. Induction of caveolar proteins was studied in isolated proximal tubules and primary renal epithelial cells. These cells were challenged with hypoxia or H₂O₂. Primary tubular cells were also subjected to viral overexpression of megakaryoblastic leukemia 1 (MKL1) and MKL1 inhibition by the MKL1 inhibitor CCG-1423. Putative coregulators of MKL1 activity were investigated by Western blotting for suppressor of cancer cell invasion (SCAI) and filamin A (FLNA). Finally, correlative bioinformatic studies of mRNA expression of caveolar proteins and MKL1 were performed.

Results

In healthy kidneys, caveolar proteins were expressed by the parietal epithelial cells (PECs) of Bowman’s capsule, endothelial cells and vascular smooth muscle. Electron microscopy confirmed caveolae in the PECs. No expression was seen in proximal tubules. In contrast, caveolar proteins were expressed in proximal tubules undergoing atrophy. Caveolar proteins were also induced in cultures of primary epithelial tubular cells. Expression was not enhanced by hypoxia or free radical stress (H₂O₂), but proved sensitive to inhibition of MKL1. Viral overexpression of MKL1 induced caveolin-1/CAV1, caveolin-2/CAV2 and SDPR/CAVIN2. In kidney tissue, the mRNA level of MKL1 correlated with the mRNA levels for caveolin-1/CAV1, caveolin-2/CAV2 and the archetypal MKL1 target tenascin C (TNC), as did the MKL1 coactivator FLNA. Costaining for TNC as readout for MKL1 activity demonstrated overlap with caveolin-1/CAV1 expression in PECs as well as in atrophic segments of proximal tubules.

Conclusions

Our findings support the view that MKL1 contributes to the expression of caveolar proteins in healthy kidneys and orchestrates the induction of tubular caveolar proteins in renal injury.

Electronic supplementary material

The online version of this article (10.1186/s12882-017-0738-8) contains supplementary material, which is available to authorized users.

Cavin-2 Functions as a Suppressive Regulator in TNF-induced Mesenchymal Stromal Cell Inflammation and Angiogenic Phenotypes

Tumour necrosis factor (TNF)-α activation of mesenchymal stromal cells (MSC) enhances their tumour-suppressive properties and tumour-homing ability. The molecular actors involved are unknown. We found that TNF induced MSC migration and tubulogenesis which correlated with a dose-dependent increase in Cavin-1 and Cavin-3 transcript levels. TNF triggered cyclooxygenase (COX)-2 expression, whereas specific siRNA-mediated gene silencing of Cavin-2 resulted in an amplified COX-2 expression, tubulogenesis, and migratory response partially due to a rapid and sustained increase in NF-κB phosphorylation status. Our results highlight a suppressive role for the caveolar component Cavin-2 in the angiogenic and inflammatory regulation of TNF-activated MSC.

Similar regulatory mechanisms of caveolins and cavins by myocardin family coactivators in arterial and bladder smooth muscle

Caveolae are membrane invaginations present at high densities in muscle and fat. Recent work has demonstrated that myocardin family coactivators (MYOCD, MKL1), which are important for contractile differentiation and cell motility, increase caveolin (CAV1, CAV2, CAV3) and cavin (CAVIN1, CAVIN2, CAVIN3) transcription, but several aspects of this control mechanism remain to be investigated. Here, using promoter reporter assays we found that both MKL1/MRTF-A and MKL2/MRTF-B control caveolins and cavins via their proximal promoter sequences. Silencing of MKL1 and MKL2 in smooth muscle cells moreover reduced CAV1 and CAVIN1 mRNA levels by well over 50%, as did treatment with second generation inhibitors of MKL activity. GATA6, which modulates expression of smooth muscle-specific genes, reduced CAV1 and CAV2, whereas the cavins were unaffected or increased. Viral overexpression of MKL1 and myocardin induced caveolin and cavin expression in bladder smooth muscle cells from rats and humans and MYOCD correlated tightly with CAV1 and CAVIN1 in human bladder specimens. A recently described activator of MKL-driven transcription (ISX) failed to induce CAV1/CAVIN1 which may be due to an unusual transactivation mechanism. In all, these findings further support the view that myocardin family coactivators are important transcriptional drivers of caveolins and cavins in smooth muscle.

Rosiglitazone drives cavin-2/SDPR expression in adipocytes in a CEBPα-dependent manner

Caveolae are abundant adipocyte surface domains involved in insulin signaling, membrane trafficking and lipid homeostasis. Transcriptional control mechanisms for caveolins and cavins, the building blocks of caveolae, are thus arguably important for adipocyte biology and studies in this area may give insight into insulin resistance and diabetes. Here we addressed the hypothesis that one of the less characterized caveolar components, cavin-2 (SDPR), is controlled by CCAAT/Enhancer Binding Protein (CEBPα) and Peroxisome Proliferator-Activated Receptor Gamma (PPARG). Using human mRNA expression data we found that SDPR correlated with PPARG in several tissues. This was also observed during differentiation of 3T3-L1 fibroblasts into adipocytes. Treatment of 3T3-L1-derived adipocytes with the PPARγ-activator Rosiglitazone increased SDPR and CEBPα expression at both the mRNA and protein levels. Silencing of CEBPα antagonized these effects. Further, adenoviral expression of PPARγ/CEBPα or Rosiglitazone-treatment increased SDPR expression in primary rat adipocytes. The myocardin family coactivator MKL1 was recently shown to regulate SDPR expression in human coronary artery smooth muscle cells. However, we found that actin depolymerization, known to inhibit MKL1 and MKL2, was without effect on SDPR mRNA levels in adipocytes, even though overexpression of MKL1 and MKL2 had the capacity to increase caveolins and cavins and to repress PPARγ/CEBPα. Altogether, this work demonstrates that CEBPα expression and PPARγ-activity promote SDPR transcription and further supports the emerging notion that PPARγ/CEBPα and MKL1/MKL2 are antagonistic in adipocytes.

Reactive oxygen species mediate angiotensin II-induced transcytosis of low-density lipoprotein across endothelial cells

The retention of plasma low-density lipoprotein (LDL) particles to subendothelial spaces through transcytosis across the endothelium is the initial step of atherosclerosis (AS). Angiotensin II (Ang II), as the principal effector molecule of the renin-angiotensin system (RAS), is implicated in several important steps of AS development. However, whether or not Ang II can directly exert a pro‑atherogenic effect by promoting LDL transcytosis across endothelial barriers, has not been defined. In the present study, we found that Ang II upregulated intracellular reactive oxygen species (ROS) levels in endothelial cells (ECs) by measuring fluorescence of 2',7'-dichlorofluorescein (DCF‑DA). Based on our transcytosis model, we observed that Ang II significantly accelerated LDL transcytosis, whereas transcytosis inhibitor methyl-β-cyclodextrin (MβCD) and ROS inhibitor dithiothreitol (DTT), markedly blocked the Ang II-stimulated increase in LDL transcytosis. Confocal imaging analysis revealed that both LDL uptake by cells and LDL retention in human umbilical venous walls were highly elevated after Ang II exposure, while MβCD and DTT significantly inhibited the effects of Ang II. What is more, proteins involved in caveolae-mediated transcytosis, including LDL receptor (LDLR), caveolin-1 and cavin-1, were associated with Ang II-induced LDL transcytosis across the ECs. Nevertheless, this process was independent of clathrin in our study. Of note, ROS inhibitor, DTT, markedly decreased the expression levels of those proteins. Consequently, ROS are critical mediators in Ang II-induced LDL transcytosis. Hopefully, these findings will provide novel insight into the crosstalk between dyslipidemia and RAS in atherogenesis.

Myocardin Family Members Drive Formation of Caveolae

Caveolae are membrane organelles that play roles in glucose and lipid metabolism and in vascular function. Formation of caveolae requires caveolins and cavins. The make-up of caveolae and their density is considered to reflect cell-specific transcriptional control mechanisms for caveolins and cavins, but knowledge regarding regulation of caveolae genes is incomplete. Myocardin (MYOCD) and its relative MRTF-A (MKL1) are transcriptional coactivators that control genes which promote smooth muscle differentiation. MRTF-A communicates changes in actin polymerization to nuclear gene transcription. Here we tested if myocardin family proteins control biogenesis of caveolae via activation of caveolin and cavin transcription. Using human coronary artery smooth muscle cells we found that jasplakinolide and latrunculin B (LatB), substances that promote and inhibit actin polymerization, increased and decreased protein levels of caveolins and cavins, respectively. The effect of LatB was associated with reduced mRNA levels for these genes and this was replicated by the MRTF inhibitor CCG-1423 which was non-additive with LatB. Overexpression of myocardin and MRTF-A caused 5-10-fold induction of caveolins whereas cavin-1 and cavin-2 were induced 2-3-fold. PACSIN2 also increased, establishing positive regulation of caveolae genes from three families. Full regulation of CAV1 was retained in its proximal promoter. Knock down of the serum response factor (SRF), which mediates many of the effects of myocardin, decreased cavin-1 but increased caveolin-1 and -2 mRNAs. Viral transduction of myocardin increased the density of caveolae 5-fold in vitro. A decrease of CAV1 was observed concomitant with a decrease of the smooth muscle marker calponin in aortic aneurysms from mice (C57Bl/6) infused with angiotensin II. Human expression data disclosed correlations of MYOCD with CAV1 in a majority of human tissues and in the heart, correlation with MKL2 (MRTF-B) was observed. The myocardin family of transcriptional coactivators therefore drives formation of caveolae and this effect is largely independent of SRF.